The Democratic Party has been hit by another cyber attack which may have compromised the details of donors.

A previously unreported cyber attack on the Democratic Congressional Campaign Committee (DCCC) is now being investigated by the FBI, sources told Reuters.

The attack may have begun in June, according to the sources. This was when a domain site with a name closely resembling a main donation site connected to the DCCC.

Internet traffic associated with donations was then sent to this bogus site.

This site apparently had an IP address similar to the hackers suspected in a previous breach of the Democratic National Committee (DNC).

This previous breach led to private emails from the DNC which were said to expose the bias within the DNC against Hillary Clinton’s main rival Bernie Sanders being released on the website Wikileaks.

DNC Chair, Representative Debbie Wasserman Schultz of Florida, resigned after the leaks were published.

Senior US. national security officials told NBC News they were confident that Russian intelligence agencies were responsible.

However, while research by cyber security company ThreatConnect showed use of Russian domains by the hackers, there is no publicly available evidence of Russian state involvement. NSA whistleblower Edward Snowden called on his former employer to release evidence publicly attributing responsibility for the hack.

The Kremlin has strongly denied involvement. A spokesperson called the allegations absurd and said that fear of Russia was being exploited for electoral purposes, in "an old trick which is being played again".

The target of the newly reported hack, the DCCC, is a fundraising body for Democrats running for seats in the House of Representatives. Information available may have included personal details of donors, such as names, email address and credit card details.

In the hands of the Democratic Party’s opponents, the details of donors could be used to make accusations about the party’s objectives and affiliations. Campaign donations have already been a major issue of the primaries to select the Democratic candidate, with Bernie Sanders criticising Clinton over accepting donations from financial services companies.

Amidst these cyber attacks, tensions between the two main political parties in the US have been high as both parties held their national conventions a week apart in July and nominated their candidates for President: Donald Trump for the Republicans and Hillary Clinton for the Democrats.

Trump appeared to call on foreign powers to hack into Hillary Clinton’s server to find around 30,000 emails that she had deleted. From 2009 to 2013 Clinton used a private address on the domain "clintonemail.com" which might have been a violation of federal recordkeeping regulations. She was investigated for this but found innocent.

"Russia, if you're listening, I hope you're able to find the 30,000 emails that are missing, I think you will probably be rewarded mightily by our press," Trump said at a press conference.

“If Russia or any other country or person has Hillary Clinton's 33,000 illegally deleted emails, perhaps they should share them with the FBI!” Trump wrote on Twitter.

He later stated that he was being sarcastic.

Hillary for America Senior Policy Advisor Jake Sullivan said: “This has to be the first time that a major presidential candidate has actively encouraged a foreign power to conduct espionage against his political opponent.

“That’s not hyperbole, those are just the facts. This has gone from being a matter of curiosity, and a matter of politics, to being a national security issue.”

The US presidential election takes place on the second Thursday in November.